An often used system for the separation of a fluid mixture, such as the cryogenic separation of air, is a double rectification column apparatus. In such a system the feed air is separated in a first column operating at a higher pressure and in a second column operating at a lower pressure wherein a main condenser serves to reboil lower pressure column bottom liquid by heat exchange with higher pressure column top vapor. The separation is driven by elevated feed pressure which is generally attained by compressing the feed in a compressor prior to introduction into the columns. The power to operate this feed compressor is the major operating cost of the separation.
The separation is carried out by passing liquid and vapor in countercurrent contact through a column. The contact is effected on vapor-liquid contacting elements which may be trays or packing. If packing is used the packing may be either random packing or structured packing. However the contacting elements cause an unavoidable pressure drop within the columns. For example, the pressure drop in the lower pressure column of an air separation plant using trays is generally within the range of from 4 to 7 pounds per square inch (psi). This column pressure drop alone constitutes about 12 percent of the compression energy power requirement of the feed compressor. Packing is known to reduce the pressure drop in the columns by a considerable amount. However, random packing generally does not have sufficient reliability for demanding separations, such as the cryogenic distillation of air, and structured packing has a very high cost.
The use of packing also causes operating problems when the air separation plant comprises a third column for the recovery of argon. In this situation a stream having a relatively high argon concentration is taken from an intermediate point of the lower pressure column and passed into the lower portion of the argon column and up the column while becoming progressively richer in argon. A crude argon product is recovered at the top of the argon column. The fluid flows are due to a pressure difference between the argon column feed stream and crude argon product stream. This pressure difference is generally about 4 psi.
Vapor product is taken from the top of the lower pressure column at a pressure slightly above atmospheric, i.e., just enough to enable the product to pass out of the plant without need for pumping. Any higher vapor product pressure would cause a separation efficiency reduction within the lower pressure column. A typical such pressure is 16.5 pounds per square inch absolute (psia). If packing is employed within the lower pressure column, the resulting low pressure drop causes the pressure at the argon column feed point to be only slightly higher than atmospheric, such as about 17 psia rather than about 20 psia when trays are used. In order to attain the requisite argon column flow with trays in the argon column, the crude argon product must be taken at a pressure about 4 psi less than the 17 psia of the argon column feed, i.e. at about 13 psia. Since this is less than atmospheric pressure, there arises the undesirable potential for air leaks into the crude argon product. This undesirable situation may be alleviated by employing packing rather than trays within the argon column but this gives rise to higher cost, if structured packing is used, or compromised reliability, if random packing is used.
It is desirable therefore to have a double column air rectification system having reduced feed compression requirements.
Accordingly it is an object of this invention to provide a double column air rectification apparatus enabling reduced feed compression requirements.
It is another object of this invention to provide a double column air rectification apparatus enabling reduced feed compression requirements without need for substantially increased cost or decreased reliability.
It is a further object of this invention to provide a double column air rectification apparatus with an argon column, enabling reduced feed compression requirements, without causing subatmospheric crude argon recovery, substantially increased argon column costs or substantially decreased argon column reliability.
It is a still further object of this invention to provide a double column air separation process having reduced feed compression requirements without need for substantially increased cost or decreased reliability.
It is yet another object of this invention to provide a double column air separation process with crude argon recovery at superatmospheric pressure without need for substantially increased cost or decreased reliability.